Torrential Rains Threaten Gulf Coast

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Water has been slopping over at least one levee in Louisiana this morning. The levee is down the Mississippi River from New Orleans, near the place where Hurricane Isaac came ashore. So far, the storm has caused street flooding along much of the Gulf Coast and left hundreds of thousands of people without power. But the full-scale of its effects will depend in part on just how long Isaac sticks around.

We begin our coverage with NPR's Greg Allen, in New Orleans.

GREG ALLEN, BYLINE: Forecasters often warn people not to focus just on the cone, the track a hurricane or tropical storm takes. The effects of a storm, wind and rain, can extend far beyond that area. The director of the National Hurricane Center, Rick Knabb, says that's especially the case with Isaac.

RICK KNABB: It's a huge storm, and that's why we could be dealing at some locations up to two days of rainfall. That could really pile up over a foot of rain.

ALLEN: The problem with Isaac is not just it's massive. It's also that it's moving slowly. Meteorologists say over the course of a couple of days, Isaac may drop seven to 14 inches of rain, with 20 inches possible in some areas.

In New Orleans, Mayor Mitch Landrieu says that may be the biggest threat posed by the storm.

MAYOR MITCH LANDRIEU: The thing that concerns us about this storm is the fact that it's going so slow, slowly, and that it could hover over the city for a long period of time. If it does that and it dumps a large amount of water on the city at a time, we expect interior flooding.

ALLEN: Landrieu says New Orleans' pumping system can get rid of an inch of rain per hour. If the rain falls faster, streets, maybe neighborhoods will flood. That could be a problem not just in Louisiana, but also in Mississippi, Alabama and in Florida's Panhandle, where the ground is already saturated from earlier rains.

Those are areas also seeing a lot of storm surge. As Isaac came ashore last night, the Hurricane Center recorded a storm surge of nearly 10 feet in Shell Beach, Louisiana, nearly six feet in Waveland, Mississippi. Meteorologists warn storm surge may continue for a day or more, as Isaac slowly moves over the area.

But Isaac won't just be a coastal event. By the weekend, Knabb says the storm system - weaker, but still very wet - is projected to move through the Midwest.

KNABB: So a lot of people could see a lot of rain and flooding. And after rains fall, you know, the river flooding can then commence.

ALLEN: Eventually, the remnants of Isaac are projected to go through Arkansas, Missouri, Tennessee and Kentucky, areas that are still recovering from last year's record flooding.